Long downloads and not getting screen burn in?

I have an imac 27 late 2012. I want to download some games from the App Store which will be over two hours. I am worried of burn in. Is there a way to put the screen a sleep and have the screen off and still have the computer download?
Can you get burn in from wallpaper or screen savers?
Will the screen saver fix burn in problems?
Is the burn in issue a real issue for all iMacs or is it hit and miss. Thanks

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